72 lines
1.7 KiB
Markdown
72 lines
1.7 KiB
Markdown
# <small>nlohmann::basic_json_document::</small>is_discarded
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```cpp
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bool is_discarded() const noexcept;
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```
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Returns whether the document holds no value, either because it was default-constructed or because the last call to
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[`parse()`](parse.md), [`parse_copy()`](parse_copy.md), or [`read()`](read.md) failed with `allow_exceptions` set to
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`#!cpp false`.
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## Return value
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`#!cpp true` if the document is discarded, `#!cpp false` otherwise.
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## Exception safety
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No-throw guarantee: this function never throws exceptions.
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## Complexity
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Constant.
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## Notes
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When the document is discarded, [`root()`](root.md) returns a discarded view (its
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[`is_discarded()`](../basic_json_view/is_discarded.md) is also `#!cpp true`).
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## Examples
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??? example
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```cpp
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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int main()
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{
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std::cout << std::boolalpha;
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// with allow_exceptions == false, a parse error produces a discarded
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// document instead of throwing -- exactly like basic_json::parse()
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json_document doc = json_document::parse(R"({"a": )", /* allow_exceptions */ false);
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std::cout << doc.is_discarded() << '\n';
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std::cout << doc.root().is_discarded() << '\n';
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// a successful parse is never discarded
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json_document ok = json_document::parse(R"({"a": 1})", false);
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std::cout << ok.is_discarded() << '\n';
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}
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```
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Output:
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```json
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true
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true
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false
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```
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## See also
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- [parse](parse.md) - deserialize from a compatible input
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- [is_discarded (basic_json_view)](../basic_json_view/is_discarded.md) - return whether a view is invalid
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## Version history
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- Added in version 3.13.0.
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